1. Field of the Invention
This invention relates to an electron beam writing data creating method and an electron beam writing data creating apparatus for creating writing data used in electron beam lithography.
2. Description of the Related Art
In the manufacture of semiconductor devices, electron beam (EB) lithography is used to expose fine patterns incapable of being exposed in photolithography. In EB lithography, EB direct writing techniques require no mask preparation for each pattern to be exposed and have thus drawn attention as a QTAT, low-cost fine pattern exposure method. Further, the character projection (CP) method using an opening of a shape of a pattern repeatedly shot enables the number of EB shots to be reduced, thus the writing throughput is improved as compared with the conventional variable shaped beam (VSB) method. Therefore, an apparatus capable of writing by not only the VSB method but also the CP method has been developed further.
In EB direct writing by the CP method, the key factor is to extract repetitive patterns (character patterns) from the pattern data for writing.
Methods of extracting character patterns include a method of creating data obtained by removing overlaps from the pattern data for writing and extracting patterns of the same shape equal to or smaller than the maximum size of the beam (normally several μm □) and a method of extracting figure cells in device pattern data by cell based design (Jpn. Pat. Appln. KOKAI Publication No. 2000-348084).
The process of extracting a character pattern is often performed at the time of data conversion. Generally, it is performed when the GDSII stream format, which is commonly used as a semiconductor device layout data format, is converted into a writing data format unique to each EB writing apparatus.
In this process, to turn figure cells into a character pattern, it is necessary to edit and modify the figure cell data before the data conversion process in order to prevent double exposure.
For example, to turn a standard cell, which is used for designing a logic device, into a character pattern, such a pattern as shown in FIG. 1 is arranged repeatedly, as shown in FIG. 2, in which the cells are arranged based on a cell layout frame. In FIGS. 1 and 2, numeral 10 indicates a cell layout frame, 21 a contact hole with no overlap, and 22 a contact hole with an overlap.
As shown in FIG. 1, when there is a pattern overlapping with the cell layout frame 10, if the entire cell is used as a character pattern, it overlaps with the adjacent patterns as shown in FIG. 2. If these overlapping patterns are used as character shots and writing is done by the CP method, as the overlapping patterns are exposed twice, the accuracy of the dimensions of the patterns is degraded.
A method of extracting a character pattern automatically from the pattern data of the figure cells which can solve double exposure problem and reduce the burden of character modifying work on the user has been proposed. In this method, a character pattern cutting frame is created from a cell layout frame included in figure cells, the figures in the character pattern cutting frame are used as the character pattern, and the character pattern is allocated to the pattern to be shot by the CP method (Jpn. Pat. Appln. KOKAI Publication No. 2005-268657).
For example, when the standard cells in FIG. 1 are used as characters, the figure cells to be used as the character patterns are separated into those inside the cell layout frame 10 and those outside the cell layout frame 10 as shown in FIGS. 3A and 3B, thereby separating the figure cells into patterns overlapping with other patterns and patterns not overlapping with other patterns. This makes it possible to create data of a character pattern which can be shot by CP method so as not to overlap with adjacent patterns.
In FIGS. 3A and 3B, 22a and 22b refer to a pattern located inside the cell layout frame 10 and a pattern located outside the cell layout frame 10, respectively, in the contact holes 22 with an overlap shown in FIG. 2. FIG. 3A shows a character pattern. FIG. 3B shows a non-character pattern.
However, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-348084, when the extracted character pattern is resized, the accuracy of the connection between the character patterns and the dimensional accuracy of connecting portion of the patterns may deteriorate.
Further, Jpn. Pat. Appln. KOKAI Publication No. 2000-348084 discloses a method for solving the problem of the accuracy of connection between the character patterns, in which the figure cells to be extracted are expanded in the upper layer, and then the character pattern are extracted.
However, in this method, since the figure cells to be extracted are expanded into the upper layer cell, the figure calculation amount at the time of extracting character patterns increases. Further, since the figure cells to be extracted are expanded into the upper layer cells, the figure cells to be extracted cannot be treated as figure cells. Therefore, the cell layout frame cannot be used as a character pattern cutting frame, which makes it impossible to extract the original figure cells.